The present invention relates to a magnetostrictive torque sensor that detects torque acting on a shaft by sensing magnetostrictive changes in magnetostrictive property.
For example, Japanese Unexamined Patent Publication No. 5-118938 and Japanese Unexamined Patent Publication No. 59-77326 disclose such magnetostrictive torque sensors.
As illustrated in FIG. 15, a prior art magnetostrictive torque sensor includes a housing 81, a shaft 83, a cylindrical magnetostrictive member 84 and a stator 85. The magnetostrictive member 84 is fitted about the shaft 83, and the stator 85 is fixed to the inner wall of the housing 81. The housing 81 is supported on the shaft 83 by a pair of bearings 82 to cover the shaft 83. The shaft 83 rotates relative to the housing 81 and the stator 85. The stator 85 is cylindrical and includes a pair of exciting coils 86 and a pair of detecting coils 87. The coils 86, 87 are located in the inner wall of the stator 85. The exciting coils 86 and the detecting coils 87 face the surface of the magnetostrictive member 84. When an alternating current is supplied to the exciting coils 86, the exciting coils 86 generate flux. The flux forms a magnetic circuit through the stator 85 and the magnetostrictive member 84.
The surface of the magnetostrictive member 84 includes two detection regions. Grooves 84a are formed in each region. The grooves 84a in one region are inclined by forty-five degrees relative to the axis, and the grooves 84a in the other region are inclined by minus forty-five degrees relative to the axis. When torque is applied to the shaft 83, a compressive force acts on one of the detection regions and a tensile force acts on the other region depending on the rotational direction of the shaft 83. A tensile force increases the magnetic permeability of the magnetostrictive member 84 and a compression force decreases the magnetic permeability of the magnetostrictive member 84. Changes in the magnetic permeability of the magnetostrictive member 84 change the voltages induced by the detecting coils 87. That is, the detecting coil 87 send varying voltage values to a processor 88. The processor 88 calculates the difference between the voltage values from the detecting coils 87. The processor 88 computes the torque applied to the shaft 83 based on the difference.
As illustrated in FIG. 15, the stator 85 is located close to the magnetostrictive member 84 to improve the sensitivity of the torque sensor. The coils 86, 87 are embedded in the inner wall of the stator 85. To facilitate the installation of the coils 86, 87, the stator 85 includes two semi-cylindrical pieces 85a. When installing the coils 86, 87, bobbins about which the coils 86, 87 are wound are attached to the inner wall of the stator 85. Thereafter, the pieces 85a are secured to each other with adhesive.
Since the stator 85 is divided into two pieces along a plane that includes the axis and since the non-conductive adhesive is located between the pieces 85a, eddy currents in the circumferential direction of the stator 85 are blocked by joints 85b.The eddy currents, which would otherwise adversely affect the sensitivity of the torque sensor, are reduced. This improves the sensitivity of the torque sensor. However, since the magnetic reluctance of the joints 85b is much greater than that of the other parts, the magnetic symmetry about the axis of the stator 85 is impaired. The lowered magnetic symmetry of the stator 85 causes the detection voltage of the torque sensor to change in accordance with the rotational position of the shaft 83. Therefore, even if there is no torque acting on the shaft 83, the torque sensor may erroneously detect that a torque is acting on the shaft 83.
Torque deforms the magnetostrictive member 84, and deformation of the member 84 changes the inductance of the coils. The torque sensor detects torque based on the changes of the inductance. However, the inductance also changes when the distance between the magnetostrictive member 84 and the coils 86, 87 changes. When the distance between the magnetostrictive member 84 and the coils 86, 87 changes, the torque sensor falsely detects torque even if there is no torque acting on the shaft 83. Therefore, it is imperative that the distance between the member 84 and the coils 86, 87 be constant to guarantee the precision of the torque sensor.
However, errors produced by assembling the stator 85 and the bearings 82 often displace the axis O1 of the shaft 83 from the axis O of the stator 85 (FIG. 17 illustrates the eccentricity in an exaggerated manner). The eccentricity causes the distance between the magnetostrictive member 84 and the coils 86, 87 to fluctuate as the shaft 83 rotates.
Due to a dimensional error created during manufacture, the cross-section of the shaft 83 may not be completely round. If the cross-section of the shaft 83 is not completely round, the distance between the magnetostrictive member 84 and the stator 85 further fluctuates, which is a further source of error.
Further, when joining the semi-cylindrical pieces 85a, the relative positions of the pieces 85a can be radially displaced relative to each other. This forms steps in the inner wall of the stator 85. The steps vary the distance between the stator 85 and the magnetostrictive member 84, which may cause the torque sensor to falsely detect torque.
To prevent false detection of torque, a dead zone, in which changes of inductance are not judged to be the result of torque applied to the shaft, has been widened. However, in an apparatus that activates an actuator based on detection of torque, such as a power steering apparatus, a widened dead zone extends the time lag from when torque starts acting on a shaft to when the actuator is activated. This deteriorates the response of the actuator.
Accordingly, it is an objective of the present invention to provide a torque sensor having an improved detectivity and sensitivity.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a torque sensor is provided. The torque sensor includes a magnetostrictive member and a stator facing the magnetostrictive member. The magnetostrictive member is strained by the torque applied to the shaft. An exciting coil and a detecting coil are accommodated in the inner surface of the stator. The exciting coil generates flux running through the magnetostrictive member. The flux varies in accordance with the strain of the magnetostrictive member. The detecting coil detects the flux variation. The cross-section of the stator""s inner surface is substantially a round. The stator includes a plurality of stator pieces. The joint between the stator pieces substantially does not face the magnetostrictive member.
The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings.